The present invention is related to curable silicone compositions. More specifically, the present invention is related to room temperature vulcanizable (RTV) adhesive formulations which provide adhesion to substrates.
Curable silicone compositions are used as laminates over a variety of substrates and in a wide variety of applications. In order to impart self-bonding adhesive properties to the silicone composition such that a primer is not needed between the silicone composition and a substrate, adhesion promoters in the silicone composition are commonly used.
Mitchell et al., U.S. Pat. No. 5,164,461, discuss an addition-curable silicone composition which includes a vinyl-containing polydiorganosiloxane, a hydrogen-terminated polysiloxane, and an adhesion promoter. The adhesion promoters include silylmaleates, silylmaleimides and silylfumarates. The silicone composition is useful for self-bonding to substrates such as plastics, metals, and glass at a cure temperature in a range between about 100xc2x0 C. and about 150xc2x0 C. The silicone composition disclosed has both excellent physical properties and excellent lap shear adhesive properties.
Stein et al., U.S. Pat. No. 5,414,066, is directed to a room-temperature addition-curable silicone adhesive composition which incorporates N-heterocyclic silanes as the adhesion promoter. The addition of the N-heterocyclic silane to a vinyl-containing polydiorganosiloxane and a hydrogen-terminated polysiloxane provides a silicone adhesive composition which cures at room temperature. The composition is directed to providing adhesion to substrates such as glass, plastics and metals.
Although silicone adhesive compositions which have the ability to cure at room temperature have been developed which provides adhesion to plastic and metal substrates, silicone adhesive compositions with new adhesion promoters are constantly being sought which can both cure at room temperature and include desired physical properties.
The present invention relates to a silicone composition which comprises a room temperature vulcanizable adhesive formulation which comprises:
(A) at least one bis(trialkoxysilylalkyl)succinate,
(B) a vinyl-containing polydiorganosiloxane,
(C) a hydrogen-containing polysiloxane, and
(D) a catalytic amount of a hydrosilylation catalyst.
A further embodiment of the present invention is a method to provide cohesive failure to a silicone composition and a substrate which comprises the steps of:
(I) applying a silicone composition to a substrate wherein the silicone composition comprises:
(A) at least one bis(trialkoxysilylalkyl)succinate,
(B) a vinyl-containing polydiorganosiloxane,
(C) a hydrogen-containing polysiloxane, and
(D) a catalytic amount of a hydrosilylation catalyst, and
(II) curing the silicone composition.
A further embodiment of the present invention provides a bis(trialkoxysilylalkyl)succinate which comprises the formula 
wherein R3 and R1 independently at each occurrence is hydrogen, an alkyl radical, aryl radical, aralkyl radical, alkaryl radical, cycloalkyl radical, or bicycloalkyl radical; R2 comprises hydrogen or an alkyl radical; and xe2x80x9cxxe2x80x9d is in a range between about 3 and about 8.
It has been found that the incorporation of an effective amount of a bis(trialkoxysilylalkyl)succinate as an adhesion promoter into silicone compositions provides a room temperature vulcanizable (RTV) adhesive formulation which adheres to metal substrates and polymer substrates. An xe2x80x9ceffective amount of a bis(trialkoxysilylalkyl)succinatexe2x80x9d as used herein is an amount of the adhesion promoter which promotes adhesion to metal and polymer substrates. Preferably, the amount of adhesion promoter is in a range between about 0.1% by weight and about 5% by weight of the total room temperature vulcanizable adhesive formulation, herein referred to as xe2x80x9ctotal compositionxe2x80x9d or xe2x80x9csilicone compositionxe2x80x9d.
The adhesion promoter is (A) at least one bis(trialkoxysilylalkyl)succinate which usually has the general formula (I): 
wherein xe2x80x9cxxe2x80x9d has a value in a range between about 3 and about 8. Each R3 and R1 independently represents hydrogen or a monovalent hydrocarbon group such as alkyl radicals, aryl radicals, aralkyl radicals, alkaryl radicals, cycloalkyl radicals, or bicycloalkyl radicals. Each R2 represents hydrogen or an alkyl radical. The term xe2x80x9calkyl radicalxe2x80x9d is intended to designate both normal alkyl and branched alkyl radicals. Normal and branched alkyl radicals are preferably those containing carbon atoms in a range between about 1 and about 20, and include as illustrative non-limiting examples methyl, ethyl, propyl, isopropyl, butyl, tertiary-butyl, pentyl, neopentyl, hexyl, octyl, decyl, dodecyl. Aryl radicals include examples such as phenyl and tolyl. Cyclo- or bicycloalkyl radicals represented are preferably those containing ring carbon atoms in a range between about 3 and about 12 with a total number of ring carbon atoms less than or equal to about 50. Some illustrative non-limiting examples of these cycloalkyl radicals include cyclobutyl, cyclopentyl, cyclohexyl, methylcyclohexyl, and cycloheptyl. Preferred aralkyl radicals are those containing carbon atoms in a range between about 7 and about 14; these include, but are not limited to, benzyl, phenylbutyl, phenylpropyl, and phenylethyl. Most preferably, a bis(trimethoxysilylpropyl)succinate is used as the adhesion promoter wherein R3 comprises an alkyl radical with 1 carbon atom, R1 comprises hydrogen, R2 comprises hydrogen, and xe2x80x9cxxe2x80x9d is 3.
The bis(trialkoxysilylalkyl)succinate of the present invention can be synthesized by first effecting a reaction between diallylsuccinate and a hydrosilylation catalyst, for example, a platinum catalyst to form a succinate-catalyst mixture. The resulting succinate-catalyst mixture is then reacted with trialkoxysilane to form the bis(trialkoxysilylalkyl)succinate.
In addition to the effective amount of at least one bis(trialkoxysilylalkyl)succinate (A), the room temperature vulcanizable adhesive formulation includes
(B) a vinyl-containing polydiorganosiloxane,
(C) a hydrogen-containing polysiloxane, and
(D) a catalytic amount of a hydrosilylation catalyst.
The vinyl-containing polydiorganosiloxane has the general formula (II),
(R4)2R5SiO[(R4)2SiO]m[R4R5SiO]nSi(R4)2R5xe2x80x83xe2x80x83(II)
wherein R5 is an ethylenic unsaturated radical, preferably vinyl; R4 is selected from the group consisting of C1-8 alkyl radicals, phenyl radicals, and C3-10 fluoroalkyl radicals and mixtures thereof, xe2x80x9cmxe2x80x9d+xe2x80x9cnxe2x80x9d has a value sufficient to provide a total vinyl-containing composition with a viscosity in a range between about 100 centipoise and about 100,000 centipoise at 25xc2x0 C., and preferably, in a range between about 3000 centipoise and about 95,000 centipoise at 25xc2x0 C. and a vinyl content in a range between about 0.02% by weight and about 2.0% by weight of the vinyl-containing polydiorganosiloxane. Radicals represented by R4 are preferably C1-4 alkyl radicals and more preferably, methyl. Typically, the vinyl-containing polydiorganosiloxane is present in a range between about 10% by weight and about 50% by weight of the total composition.
The vinyl-containing polydiorganosiloxane (B) includes (1) the vinyl-containing polydiorganosiloxane and may also include (2) a vinyl-containing siloxane resin copolymer.
The vinyl-containing siloxane resin copolymer is present in a range between about 0% by weight and about 70% by weight of the total composition having
(R6)3SiO1/2 units (xe2x80x9cMxe2x80x9d) and SiO4/2 units (xe2x80x9cQxe2x80x9d),
wherein R6 is a vinyl radical, or a monovalent hydrocarbon radical free of aliphatic unsaturation and containing no more than six carbon atoms, the ratio of (R6)3SiO1/2 units to SiO4/2 units being in a range between about 0.5:1 and about 1.5:1, and the resin having a vinyl content in a range between about 1.5% by weight and about 3.5% by weight of the vinyl-containing siloxane resin copolymer. The vinyl-containing siloxane resin copolymer is also referred to as the xe2x80x9cvinyl-containing MQ resinxe2x80x9d or xe2x80x9cMViQxe2x80x9d.
The vinyl-containing siloxane resin copolymer may further contain (i) R6SiO3/2 units (xe2x80x9cTxe2x80x9d), (ii) (R6)2SiO2/2 units (xe2x80x9cDxe2x80x9d), or combinations thereof, where the (R6)2SiO2/2 units are present in an amount in a range between about 0 mole percent and about 10 mole percent based on the total number of moles of siloxane units in the vinyl-containing siloxane resin copolymer and R6 is as defined above.
The hydrogen-containing polysiloxane (C) functions as a cross-linking agent and is typically present in a range between about 1% by weight and about 15% by weight of the total composition. The hydrogen-containing polysiloxane is represented by an average unit formula (III),
R7dHeSiO(4xe2x88x92dxe2x88x92e)/2xe2x80x83xe2x80x83
wherein R7 is hydrogen, a monovalent hydrocarbon radical, or halogenated monovalent hydrocarbon radical having carbon atoms in a range between about 1 and about 10, and free of aliphatic unsaturation, xe2x80x9cdxe2x80x9d has a value in a range between 0 and about 3, xe2x80x9cexe2x80x9d has a value in a range between about 1 and about 3, and the sum of xe2x80x9cdxe2x80x9d+xe2x80x9cexe2x80x9d has a value in a range between about 1 and about 3. R7 is preferably selected from C1-8 alkyl radicals, phenyl, C3-10 fluoroalkyl radicals, and hydrogen and most preferably, methyl. Most typically, the preferred fluoroalkyl radical is trifluoropropyl.
A preferred hydrogen-containing polysiloxane has the formula (IV) 
where R7 is as defined above, xe2x80x9cfxe2x80x9d and xe2x80x9cgxe2x80x9d have values which are sufficient when added together to provide a viscosity in a range between about 10 centipoise and about 1000 centipoise at 25xc2x0 C., and preferably, in a range between about 10 centipoise and about 150 centipoise at 25xc2x0 C., and the reactive hydrogen content is in a range between about 0.02% by weight and about 1.6% by weight of the hydrogen-containing polysiloxane. The hydrogen-containing polysiloxane of formula (IV) can be used as a hydride cross-linking agent in the present invention.
Other hydrogen-containing polysiloxanes which can be used in the present invention include siloxane copolymer resins comprised of xe2x80x9cMxe2x80x9d units which comprise (R7)3SiO1/2, xe2x80x9cMHxe2x80x9d units which comprise H(R7)2SiO2, xe2x80x9cDxe2x80x9d units which comprise (R7)2SiO2/2, xe2x80x9cDHxe2x80x9d units which comprise HR7SiO2/2, xe2x80x9cTxe2x80x9d units which comprise R7SiO3/2, xe2x80x9cTHxe2x80x9d units which comprise HSiO3/2, and xe2x80x9cQxe2x80x9d units, and mixtures thereof wherein the mixtures comprise at least one hydrogen. The preferred resins are known as MHQ resins, which comprise diorganohydrogensiloxy units (MH) and SiO4/2 units (Q) wherein the ratio of diorganohydrogensiloxy units to Q units is in a range between about 0.4:1.0 and about 2.0:1.0 inclusive. Hydrogen-containing polysiloxanes having at least one R7 group, preferably, a methyl group, bonded to silicon which bears at least one reactive hydrogen atom are preferred. It is understood that the hydrogen-containing polysiloxane can be a single compound or a mixture of compounds. Additional hydrogen-containing polysiloxanes suitable for use in the present invention are described, for example, in U.S. Pat. No. 4,061,609.
The room temperature vulcanizable adhesive formulation further contains a catalytic amount of a hydrosilylation catalyst (D). The hydrosilylation catalyst (E) promotes the hydrosilylation reaction. The hydrosilylation catalyst (D) typically is a platinum group metal catalyst. Additional catalysts for facilitating the hydrosilylation curing reaction include precious metal catalysts such as those containing ruthenium, rhodium, palladium, osmium, or iridium, or complexes of these metals. Examples of suitable hydrosilylation catalysts for use in the present invention are disclosed, for example, in U.S. Pat. Nos. 3,159,601 and 3,159,662; 3,220,970; 3,814,730; 3,516,946; and 4,029,629.
The hydrosilylation catalyst is preferably a platinum-containing catalyst. Preferably, the platinum-containing catalyst is a platinum complex formed by reacting chloroplatinic acid containing about 4 moles of water of hydration with tetravinylcyclotetrasiloxane in the presence of sodium bicarbonate in an ethanol solution. This catalyst is disclosed in U.S. Pat. No. 3,775,452 and is often referred to as Karstedt""s catalyst.
The hydrosilylation catalyst is used in a catalytic amount, which is an amount sufficient to promote the hydrosilylation reaction. Generally, there is utilized at least about 0.1 part per million (ppm) of a platinum catalyst, and preferably in a range between about 5 parts per million and about 250 parts per million, in terms of parts of platinum metal based on the weight of total composition.
Inhibitors such as acetylenic alcohols (e.g., 3,5 dimethyl-1-hexyn-3-ol and 2 methyl-3-butyn-2-ol), amines, and tetravinyltetramethylcyclotetrasiloxane and mixtures thereof can also be employed when used in an effective amount which is typically in a range between about 0.01% by weight and about 1% by weight of the total composition.
The room temperature vulcanizable adhesive formulation of the present invention may also contain any of the conventional extending fillers (E), reinforcing fillers (F), and mixtures thereof. The room temperature vulcanizable adhesive formulation contains extending filler in a range between about 0% by weight and about 50% by weight, and preferably in a range between about 10% by weight and about 30% by weight of the total composition, and reinforcing filler in a range between about 0% by weight and about 70% by weight, and preferably in a range between about 20% by weight and about 50% by weight of the total composition.
Examples of extending fillers (E) useful herein include alpha quartz, crushed quartz, aluminum oxide, aluminum silicate, zirconium silicate, magnesium oxide, zinc oxide, talc, diatomaceous earth, iron oxide, calcium carbonate, clay, titania, zirconia, mica, glass, such as ground glass or glass fiber, sand, carbon black, graphite, barium sulfate, zinc sulfate, wood flour, cork, fluorocarbon polymer powder and the like. Alpha quartz is the most preferred extending filler.
Examples of reinforcing fillers (F) include silica, such as fumed silica or precipitated silica, and treated silica fillers such as fumed or precipitated silica that has been reacted with, for example, an organohalosilane, a disiloxane, or a disilazane. Fumed silica is particularly effective as a reinforcing filler for the compositions of the present invention. A particularly preferred treated fumed silica is one wherein the fumed silica has been treated first with cyclic polysiloxanes, for example, dimethylcyclic tetramer, according to the methods as described in U.S. Pat. No. 2,938,009, and then treated with a silazane, for example, hexamethyldisilazane, as described in U.S. Pat. Nos. 3,635,743 and 3,847,848, so as to remove most of the free silanols on the surface of the tetramer treated silica. Removal of most of the free silanols refers to less than about 30% silanols remaining on the surface of the tetramer treated silica. Such a filler is sometimes referred to herein as xe2x80x9ctreated fumed silicaxe2x80x9d.
The room temperature vulcanizable adhesive formulation of the present invention typically is prepared by homogeneously mixing (i.e. uniformly mixing) components (B)-(E) with the adhesion promoter (A) and any optional ingredients, using suitable batch, continuous, or semi-continuous mixing means, such as a spatula, a drum roller, a mechanical stirrer, a three-roll mill, a sigma blade mixer, a bread dough mixer, or a two-roll mill.
It is possible to mix all components in one mixing step immediately prior to the intended use of the curable composition. Alternatively, certain components can be pre-mixed to form two or more packages which can be stored, if desired, and then mixed in a final step immediately prior to the intended use thereof.
Preferably, the vinyl-containing polydiorganosiloxane (B) will be homogeneously mixed with a hydrosilylation catalyst (D) and any additional reinforcing filler to form a package (1). Package (2) will be a mixture of the hydrogen-containing polysiloxane (C), at least one vinyl-containing polydiorganosiloxane, and inhibitor. Package (2) is modified with adhesion promoter, (A), additional inhibitor, and additional hydrogen-containing polysiloxane. Package (1) and package (2) are then homogeneously mixed. Typically, the weight ratio of package (1) to package (2) is in a range between about 15:1 and about 1:1 and preferably, in a range between about 12:1 and about 1:1. These two packages can then be stored until the composition of the present invention is desired and then homogeneously mixed.
The thickness of the total composition on a substrate is typically in a range between about 20 millimeters and about 60 millimeters. The total composition of the present invention can be applied to the surface of a substrate by any suitable means such as rolling, spreading, spraying, and the like, and subsequently cured. After application of the total composition onto the substrate, the composition can be cured at room temperature over a period in a range between about 0.25 hours and about 150 hours. Typically, the cure temperature is in a range between about 25xc2x0 C. and about 150xc2x0 C.
When applied to a substrate, the total composition of the present invention has the desirable property of failing cohesively instead of adhesively when tested. A lap shear adhesion strength test is commonly used to measure adhesive failure and cohesive failure. xe2x80x9cAdhesive failurexe2x80x9d as used herein indicates that the silicone layer separates from the substrate at the point wherein the two layers meet, that is, the bond between the silicone layer and the substrate ruptures before the silicone layer or the substrate ruptures. xe2x80x9cCohesive failurexe2x80x9d as used herein indicates that the silicone layer or the substrate ruptures before the bond between the silicone layer and the substrate fails.
The room temperature vulcanizable adhesive formulation has been found to cohesively bond on metal substrates, metal oxide substrates, and polymer substrates. Metal substrates include, but are not limited to, treated and untreated aluminum substrates and treated and untreated steel substrates. Polymer substrates include, but are not limited to phenolic resins, epoxy resins, polyetherimides, polyamides, unsaturated polyesters, poly(ethylene terephthalate), polycarbontes, polyphenylene sulfide, polyacetals, and polyimides. Preferably, the room temperature vulcanizable adhesive formulation is used for adhesion to phenolic resins, epoxy resins, and untreated metal substrates. Treating the substrate may include cleaning the substrate, applying a primer known to those skilled in the art, or a combination thereof. The compositions can be used as adhesives for applications in the military and laser industry as well as the electronic industry and automotive industry.
Room temperature vulcanizable adhesive formulations cure by mechanisms such as hydrosilylation or condensation. In order that those skilled in the art may better understand the practice of the present invention, the following examples of silicone compositions curing via hydrosilylation are given by way of illustration and not by way of limitation.